Navigation systems are increasing dependency upon the global positioning system (GPS). The navigation receiver should be able to withstand intentional or unintentional signal interference with robust signal acquisition and tracking. For this reason, inertial navigation systems have been coupled to GPS signal tracking and acquisition systems for improved signal tracking and acquisition of the received signal. Inertial navigation systems (INS) have an inertial measurement unit (IMU) for processing inertial measurements. Coupling navigation data with the GPS signal tracking and acquisition system improves signal tracking and acquisition. When the inertial measurements of a navigation processor are used with a GPS signal tracking and acquisition system, the combined system is said to be coupled. The tightly coupled GPS and INS system uses validated pseudorange and pseudorange rate measurements from the GPS receiver instead of the processed position and velocity measurements. In addition, inertial measurement unit data is usually used to assist the receiver tracking loops and enable more noise filtering than would be possible without tracking loop aiding.
The tightly coupled systems have been used with validated pseudorange and pseudorange rate measurements. More recently, ultratight GPS and INS coupling has been proposed which drives all signals from the INS navigation solution for a period of time, and then, examines all signals with respect to the error of the navigation solution. Ultratight receivers do not attempt to control code replicas for exact code phase alignment with the incoming signal but merely seek to observe the deviation of the incoming signal from locally generated replicas and feed that deviation information back to the code replica generators. Ultratight coupling has shown to be more robust in the presence of noise. Ultratight coupling does not require that the replica code track or lock onto the incoming signal. Ultratight coupling systems have been disclosed in U.S. Pat. No. 6,516,021. Observation lock detectors have been developed to extend the concept of lock detection to ultratightly coupled systems. The observation lock detectors enables error measurements to be validated before applying the error measurement to the navigation state estimate, and provides a metric of system performance.
All GPS receivers whether uncoupled, loosely coupled, tightly coupled, or ultratightly coupled correlate inphase and quadrature early E, prompt P, and late L, GPS signal replicas with the incoming GPS signal resulting in early, prompt, and late quadrature correlations that are denoted as IE, QE, IP, QP, IL, QL. These quadrature correlations have a nonlinear relationship to the difference between each replica code or carrier phase and that of the incoming received signal. The quadrature correlations are therefore used to determine the difference in the replicas code and carrier phase and that of the incoming signal. The difference between the replica code or carrier phase and that of the incoming signal is used to refine the estimate of the navigation state solution because the code and carrier phase of the incoming signal are related to the true range from the receiver to the transmitting satellite as the code and carrier phase of the replicas are related to the estimated range from the receiver to the satellite.
In ultratightly coupled receivers, a prefilter is typically a sequential filter that filters the quadrature correlation outputs to estimate one or more quantities such as a pseudorange error Δρ, pseudorange error rate Δ{dot over (ρ)}, a pseudorange error acceleration Δ{umlaut over (ρ)}, carrier phase error φ, a carrier frequency error φ′, carrier phase acceleration error φ″, a total electron content error ΔTEC, a total electron content error rate ΔTEC′, as well as a measurement covariance matrix M. The output of the prefilter is received by a lock detector. When the estimates are valid, then the estimates are forwarded to the navigation state estimator as measurements for determining the navigation solution.
A plurality of nonlinear transforms of the quadrature correlation outputs, called discriminators, are often used in tracking loop systems. Three types of discriminators are commonly used: carrier phase discriminators, carrier frequency discriminators, and code discriminators. The carrier phase discriminators may simply be the quadrature correlation outputs or a product of the quadrature correlation outputs, which results in the data bits being stripped off. The carrier phase discriminators provide the most information, but the carrier phase discriminators are also the most nonlinear so that carrier phase information may not be easily extracted by a sequential filter. When all of the epoch information can be extracted from the carrier phase discriminators by the sequential filter, there is no need to use the other carrier frequency discriminators or the code discriminators. The phase discriminators contain the most epoch information of received signal epoch timing. The carrier frequency discriminator's functions of the correlator outputs possess no carrier phase information but do contain carrier frequency information. The carrier frequency discriminators do not provide as much epoch information as the carrier phase discriminator. However, the carrier frequency discriminator provides information that is more readily extracted by a sequential filter. Sequential filter processing of epoch timing information from carrier frequency discriminators is less accurate than a sequential filter processing of epoch timing information from carrier phase discriminators, but the sequential filter processing of epoch timing information from carrier frequency discriminators is more robust. The code discriminators provide the least epoch information, but that epoch information provided is the most easily extracted by a sequential filter. A sequential filter processing of epoch timing information from the code discriminators is the most robust but less accurate than sequential filters processing of epoch timing information from the carrier phase discriminators or the carrier frequency discriminators. Existing conventional tracking loop systems use multiple discriminators in conjunction with lock detectors to ensure that the most accurate epoch timing information is being processed only when valid. The less accurate discriminators are used only when the epoch information from the carrier phase discriminators cannot be reliably extracted. Multiple sequential prefilters have been proposed to solve the multipath problem. However, each of the sequential filters employed corresponds to a different multipath assumption. Parallel sequential prefilters have been used. Each of the parallel sequential prefilters have a different assumed noise level and apply the prefilters to the same I&Q correlations. As such, existing systems use a signal discriminator as input to the sequential filters. Consequently, existing navigation systems do not ensure that the most accurate and reliable epoch timing information is being extracted, validated, and used to compute the navigation solution. These and other disadvantages are solved or reduced by the present invention.